Antenna Driving me nuts
charles copeland
Chris Valliant <Chr...@valteck.com> wrote:
>
>
> I have been trying for the last two weeks to build a half wave
>diapole antenna. I have read the book twice, done my calculations down to
>the half inch, used several different gauges of wire, several makes of
>coax cable but I cannot get the SWR reading below 6:1. Even with an
>antenna tuner I cannot get an acceptable SWR reading. It's driving me to
>the funny farm. I have about a mile of wire in my apartment and have run
>out of ideas. Here is what I am trying to do: to build a 1/2 wave
>diapole for the 28-29 mhz. I splice the coax, solder one end to a length
>of wire 8.2 feet and the other braided end to a length of wire the same
>size. I suspend both ends using insulated clamps between two tree's,
>making sure that the whole antenna is parallel to the ground and it's 8
>feet above the ground. Now when I check the SWR, it's unbelieveable. Like
>I have said before, I have used different wire, different coax (the
>standard coax, just different brands). What am I doing wrong ? Can anyone
> spot my mistake ? I will gladly make it up to whomever spots my error.
>Anyway, thanks for listening.
Swtich from coax to twin lead. You can buy ladder line from ham shop
or just use TV antenna twin lead wire. Feed it into your balance line
input to the tuner.
I had the same type of problems with SWR till I did this on my dipole.
Dave Potter
>
Here is what I am trying to do: to build a 1/2 wave
>diapole for the 28-29 mhz. I splice the coax, solder one end to a length
>of wire 8.2 feet and the other braided end to a length of wire the same
>size. I suspend both ends using insulated clamps between two tree's,
>making sure that the whole antenna is parallel to the ground and it's 8
>feet above the ground.
Chris -
I'm not an antenna expert, but I'd look at the following:
1) It's my understanding that if you just attach the center conductor
and shield to the two halves of the antenna directly, your feed line
will become part of the antenna, and it will radiate. Your transmitter
may be looking at an antenna a whole lot longer than just the two
8 foot sections. That would explain the high SWR. Better to get
a balun which has screw connectors for the two antenna pieces, and a
SO-239 connector for your feedline. I think a 1:1 balun would work,
but I could be wrong on that.
2) Height may be part of the problem. 8 feet high isn't much.
3) What do you mean "insulated clamps"? I'd get some insulators and
suspend the antenna by some rope or wire, tying the suspension wire to
one side of the insulator, and the antenna section to the other end of
the insulator. That's the way I've seen dipoles hung before.
4) Parallel to the ground isn't as critical as some of these other
things, I think.
5) Why 28-29 MHZ? The band's dead.
Bill Nelson
: I have been trying for the last two weeks to build a half wave
: diapole antenna. I have read the book twice, done my calculations down to
: the half inch, used several different gauges of wire, several makes of
: coax cable but I cannot get the SWR reading below 6:1. Even with an
: antenna tuner I cannot get an acceptable SWR reading. It's driving me to
: the funny farm. I have about a mile of wire in my apartment and have run
: out of ideas. Here is what I am trying to do: to build a 1/2 wave
: diapole for the 28-29 mhz. I splice the coax, solder one end to a length
: of wire 8.2 feet and the other braided end to a length of wire the same
: size. I suspend both ends using insulated clamps between two tree's,
: making sure that the whole antenna is parallel to the ground and it's 8
: feet above the ground. Now when I check the SWR, it's unbelieveable. Like
: I have said before, I have used different wire, different coax (the
: standard coax, just different brands). What am I doing wrong ? Can anyone
: spot my mistake ? I will gladly make it up to whomever spots my error.
The general method is to make the wires too long (by several feet), and
then trim them for resonance. Have you tried that? If you trim the
original length, and the VSWR goes up - then your calculations are wrong,
and the starting lengths are too short.
You should be able to match just about anything, using a tuner - assuming
that it is a manual box, and not an automatic tuner, they generally have
problems starting at about 3:1 or 4:1.
The easiest to match is an "inverted V", and that can be used without a
tuner, as the impedance is about 50 ohms (or close enough that it does
not matter.
Get it up very high in the air, as horizontal antennas are very sensitive
to height above ground (for low heights). The feedpoint impedance can vary
from about 2 ohms to about 100 ohms - just from varying the height.
The dipole would best be fed using a current balun at the feedpoint.
Otherwise, your coax can act as part of the antenna - and all bets are
off for resonating the antenna.
Bill
Bill Nelson
: 5) Why 28-29 MHZ? The band's dead.
It is not dead, by a long shot. The openings are a lot rarer, but around
the world contacts are still possible. It is rather nice, as there are so
few people on the band - little qrm or competition for contacts.
Bill
Consumer Electronics Specialists
>
> Chris Valliant (Chr...@valteck.com) wrote:
>
> : I have been trying for the last two weeks to build a half wave
> : diapole antenna. I have read the book twice, done my calculations down to
> : the half inch, used several different gauges of wire, several makes of
> : coax cable but I cannot get the SWR reading below 6:1. Even with an
> : antenna tuner I cannot get an acceptable SWR reading. It's driving me to
> : the funny farm. I have about a mile of wire in my apartment and have run
> : out of ideas. Here is what I am trying to do: to build a 1/2 wave
> : diapole for the 28-29 mhz. I splice the coax, solder one end to a length
> : of wire 8.2 feet and the other braided end to a length of wire the same
> : size. I suspend both ends using insulated clamps between two tree's,
> : making sure that the whole antenna is parallel to the ground and it's 8
> : feet above the ground. Now when I check the SWR, it's unbelieveable. Like
> : I have said before, I have used different wire, different coax (the
> : standard coax, just different brands). What am I doing wrong ? Can anyone
> : spot my mistake ? I will gladly make it up to whomever spots my error.
>
Its been a few years, but I had the same problem when I was a new
novice.
I made a combonation 40 and 80 M dipole, and the swr was to the moon!
After weeks of pruning 1/2" at a time, I finally decided to try a
different (cb)
swr meter. As I was un-hooking the suspected 'bad' meter, I discovered
it was
hooked up backwards (OOPS). I hooked it up correctly and presto.
It sounds simple, but in the heat of battle its easy to fudge-up a
connection.
73 de KD9SR
Sir Barry Duxbury
char...@netcom.com (charles copeland) writes:
> In article <4qsk9b$s...@news.inforamp.net>,
> Chris Valliant <Chr...@valteck.com> wrote:
> >
> >
> > I have been trying for the last two weeks to build a half wave
> >diapole antenna.
Just a general observation. This thread appears to tell us a lot
about the US licensing standards. In 40+ years I have never seen
anyone having a problem with a simple dipole or the underlying basic
principles. What has really made me comment is the pattern of very
elementary questions which appear on the net frm licencesed hams. I
accept thar Chris may not be a ham and certainly this comment is not
critical of him.
73 Barry
--
Barry Duxbury
bdux...@zetnet.co.uk
100031.2223@compuserve
G4GAH Oxford UK
Bill Nelson
Consumer Electronics Specialists (c...@netnitco.net) wrote:
: Bill Nelson wrote:
: >
: >
: > : I have been trying for the last two weeks to build a half wave
... rest deleted to save bandwidth ...
: Its been a few years, but I had the same problem when I was a new
: novice.
I did not write any of the above, Chris did. My name should NOT be in
the attributions.
Bill
: 73 de KD9SR
Chris Broadbent
Bill Nelson wrote:
>
> ...<SNIP>
>
> The dipole would best be fed using a current balun at the feedpoint.
> Otherwise, your coax can act as part of the antenna - and all bets are
> off for resonating the antenna.
>
> Bill
This is a different Chris, I'm the one working on a 50W 40/80M CW TX design
(I've posted other questions already on some of the Ham groups).
I have read that for a 50 Ohm feed to a simple 40M or 80M suspended half
wave dipole, a balun is not necessary (but can help). Also, a couple of
Hams recommend that I avoid a balun if possible for a TX, as it sometimes
introduces quirks of its own.
Am I mistaken in assuming I can just feed the half wave dipole with RG8/U,
without a balun? Would a balun be helpful, or more trouble than it's
worth, given the above information?
Cheers,
Chris. ( who's studying for the tests while working on the TX design ).
Alexander Vrenios
>> Chris Valliant (Chr...@valteck.com) wrote:
>>
>> : I have been trying for the last two weeks to build a half wave
>> : diapole antenna. I have read the book twice, done my calculations down to
>> : coax cable but I cannot get the SWR reading below 6:1. Even with an
>> : antenna tuner I cannot get an acceptable SWR reading. It's driving me to
>> : the funny farm. I have about a mile of wire in my apartment and have run
>> : out of ideas. Here is what I am trying to do: to build a 1/2 wave
>> : diapole for the 28-29 mhz. I splice the coax, solder one end to a length
>> : of wire 8.2 feet and the other braided end to a length of wire the same
>> : size. I suspend both ends using insulated clamps between two tree's,
>> : making sure that the whole antenna is parallel to the ground and it's 8
>> : feet above the ground. Now when I check the SWR, it's unbelieveable. Like
>> : I have said before, I have used different wire, different coax (the
>> : standard coax, just different brands). What am I doing wrong ? Can anyone
>> : spot my mistake ? I will gladly make it up to whomever spots my error.
Left Half Right Half
-----------------------\ /-----------------------
\ | <--- Center conductor
| ---
| | |
| | | <--- Center insulator (often white)
Braid ---> \\| |
-------
| |
| | <--- Outer shield (often black)
| |
| | <--- Coax (something
| | like RG8/U type)
| |
| |
| |
|-----|
/ \
Screw-on Ring --->| | <--- PL-259 Connector
--------- (Also called a
Center Pin --->| UHF connector)
|---o---|
| | <--- SO-239 on back of transmitter
-------------
Left and right halves of dipole antenna are of equal lengths. These
two equal lengths should add up to the halfwave length of the frequency
at which you wish to operate. For example, if you wish to operate the
40 Meter band, at 7.150 MHz, divide that value into 468, resulting in
the total length of a dipole for 7.150 MHz, in feet:
468
----- = 65.45 Feet
7.150
This being approximately 65 1/2 feet, I would cut the two equal halves
to be 32 feet, 8 inches each. I would expect the SWR to measure well under
the 6:1 you reported earlier.
I hope this helps you; these are, of course my opinions only: this is
what I would do in your place.
One final note. I have had more trouble with SWR meters than I've had
with dipole antennas. That is, if you know you are correct, you might want
to ask someone to help you measure the SWR of your new antenna. (I can't
immagine anyone in this hobby refusing such a simple request, especially
since you've already got the antenna up and own a meter!) Best of luck,
and try not to let this negative experience influence you too much.
Regards,
Alex Vrenios Distributed Algorithms
Vre...@asu.edu _._ _.._ ____. .. Research Lab
Ed Wright
In article <4r0u68$b...@xenon.brooks.af.mil>,
Dave Potter <Dave....@guardian.brooks.af.mil> wrote:
>Chr...@valteck.com (Chris Valliant) wrote:
>>
> Here is what I am trying to do: to build a 1/2 wave
>
10 opens up from time to time. Kind of makes it a treat.
Works good for local rag chews.... doesn't have to be dx to be fun.
de KA9AHQ/7
Ed
--
I think I've got the hang of it now .... :w :q :wq :wq! ^d X exit ^X^C ~. ^[x X Q :quitbye CtrlAltDel ~~q :~q logout save/quit :!QUIT ^[zz ^[ZZ ZZZZ ^H ^@ ^L ^[c $q ^# ^E ^X ^I ^T ? help helpquit ^D ^d ^C ^c help ^]q exit ?Quit ?q
\qy \xyy F.M.H. e...@sequent.COM e...@teleport.com
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ED WELCH
-> Here is what I am trying to do: to build a 1/2 wave >diapole for the
-> 28-29 mhz. I splice the coax, solder one end to a length
-> 5) Why 28-29 MHZ? The band's dead.
Oh, not completely dead. I've been picking up several QSO's for several
days now. 'Caught a NE net the other evening, and a guy CQing out of
Quebec. Waiting on my ticket to arrive I haven't responded to any of
these. Maybe crippled, but certainly not dead.
Chris might be like me, just getting his feet wet in radio and wanting
to be ready when the next cycle kicks in. Maybe by then I'll be a more
skilled operator. Digging weak signals out now could help in the
future.
I agree that 10m ain't hopping, but as I write this I'm copying a guy in
northern Ohio on 28.47010...I'm down in south Alabama. Worldwide, no,
but workable.
Just my two cents worth. :)
Bob Bruhns
Seems to me that if the antenna is fed exactly in the center, 1/2
of the feed voltage would be applied to the coax shield if no balun is
used. BUT, it should be possible to find a point slightly off center
where the voltage could balance out on the shield side of the coax,
meaning no antenna current on the shield. If the "shield" side of the
dipole is slightly longer than the center-conductor side - one or two
inches of difference should do it on 80 and 40 meters - then the
necessary balance should be attainable.
The problem is that this degree of precision in wire length is not
likely to be achieved in a typical amateur dipole. We trim a few feet
here and there to minimize SWR, and we lose the one inch...
Perfect or near-perfect balance may occur in some cases, but without a
balun it's a crapshoot unless you calibrate the wire ends... and even
then, nearby conductors can upset the balance. So, it's either a balun,
or it's a current meter on a stick, and careful tweaking for balance.
Bob Bruhns, WA3WDR, bbr...@li.net
Jim Reid
> Chris Valliant (Chr...@valteck.com) wrote:
>
> : I have been trying for the last two weeks to build a half wave
> : diapole antenna. I have read the book twice, done my calculations down to
> : the half inch, used several different gauges of wire
Your antenna is being loaded by capacitive reactace from
the surroundings: only 8 feet abouve ground, the two
trees, trunks and limbs, humidity in the air, etc.
The formula you have used to calculate the length assumes
there are no external sources of capacitance to lower the
resonant frequency of the wire dipole. The resonant frequency
is inversely proportional to the square root of the product
of the capacitance and inductance of all factors about your
antenna and its installed environment. So if you have a
swr of 6:1 at whatever frequency you are testing, instead
of a 50 ohm effective radiation resistance, it is acurally
either about 8 ohms or 300 ohms ( that is, either 6 x 8= 48,
close to 50; or 6 x 50 = 300 ohms). And it is probably the
first, or around 8 ohms. You may deal with this using an
antenna tuner, or adjust the length of the two legs of the
dipole to bring the radiation resistance back up towards 50.
In order to determine where your antenna is now resonant, or at
least in which direction from where you are measuring it now,
which I assume is somewhere in the middle of the band of
interest, go to the lower end of the band and see if the swr
has dropped at all. If it has, then the antenna is too long,
and is resonating well below the band. So, try snipping off
a foot from each end; check the swr again to see if it has
dropped. If so, you could now calculate how much more to
snip off, equally from each leg, to bring it right up to
where you want it, but that takes some calculater arithmatic,
so just keep snipping equally until it has the lowest possible
swr at the frequency of interest. It may or may not reach a
swr of 1.0:1, since your antenna may not be exactly 50 ohms
at resonance; in fact a dipole free of any external reactance
factors, will have a resonant resistaance of about 72 ohms,
which is why I feed all four of my dipoles using 72 ohm balance
transmission line (available from Van Gordon Engineering).
I know it is a lot of work to keep bringing the dipole to
ground level to snip it; oops, snipped too mnuch, and then have to
solder a couple of pieces back on, but it works, just
uses up a Saturday afternoon or morning!
73 abd Good Luck, Jim, AH6NB
On the Garden Island of Kauai
Jim Reid
Chris Broadbent wrote:
> I have read that for a 50 Ohm feed to a simple 40M or 80M suspended
>half wave dipole, a balun is not necessary (but can help).
> Am I mistaken in assuming I can just feed the half wave dipole with
>RG8/U, without a balun? Would a balun be helpful, or more
> trouble than it's worth, given the above information?
A 1:1 balun may be used to insure that rf current does not flow
on the outer coax shield. Probably it will not improve the
swr of the antenna system. A 1:1 balun will transfer to its
output connector whatever impedance/resistance it sees at its
input connector. For a half wave dipole in most of our
set ups, this won't be 50 ohms since the dipole will be
seeing capacitive reactance coupled in from the surrounding
environment. The purpose of the balun not allowing current
to flow on the outer coax shield, is to prevent those
rf fields so radiated from unbalancing the dipole and
modifying the expected radiation pattern. Has nothing to
do with the resonance nor the radiation efficiency of the dipole
operation.
Actually, a half wave dipoole, installed totally free
of any surrounding reactance, will have a radiation
resistance at resonance of about 72 ohms. For this reason,
Big Guns using high power have been known to feed them with
high power, that is, high current carrying capacity 72 ohm
balanced twin-lead transmission line. This line has conductors
near 12 gauge wire in diameter (available from Van Gordon
Engineering, among other places, mfg by Amelco). In the
ARRL handabooks this line is shown to have the lowest
losses of any other transmission line in the HF frequency
ranges.
To couple this line to a 50 ohm output of nearly every
rig today, use an Amidon, 1.5:1 balun. This matches
the rig to the about 70+ ohm radiation resistance of the
described dipole set up. With this set up, at the resonant
operating frequency, the vswr meter at your rig will
read almost a perfect 1.0:1, since all elements are
about perfectly matched without using an antenna tuner.
Hope my long winded words help some in you understanding
of what is going on.
73 and Aloha, Jim, AH6NB
Bill Turner
Chris Broadbent <c...@bga.com> wrote:
<snip>
>This is a different Chris, I'm the one working on a 50W 40/80M CW TX design
>(I've posted other questions already on some of the Ham groups).
>
>Hams recommend that I avoid a balun if possible for a TX, as it sometimes
>introduces quirks of its own.
>
>without a balun? Would a balun be helpful, or more trouble than it's
>worth, given the above information?
>
In putting up lots of dipoles over the years, I've only found one
situation where a balun really helped. That was when running high
power (over about 400 watts in my case) and I had problems with RF
getting back into my keyboard. I had tried all kinds of filtering at
the keyboard/computer, but the only thing that really killed it was a
balun at the apex of my 40 meter inverted vee. Now, even at 1500
watts, there in no interference at all.
73, Bill W7LZP
w...@eskimo.com
Peter Reed
ED WELCH (ed.w...@cheaha.com) wrote:
: -> Here is what I am trying to do: to build a 1/2 wave >diapole for the
: -> 28-29 mhz. I splice the coax, solder one end to a length
: -> 5) Why 28-29 MHZ? The band's dead.
: Oh, not completely dead. I've been picking up several QSO's for several
: days now. 'Caught a NE net the other evening, and a guy CQing out of
: Quebec. Waiting on my ticket to arrive I haven't responded to any of
: these. Maybe crippled, but certainly not dead.
: Chris might be like me, just getting his feet wet in radio and wanting
: to be ready when the next cycle kicks in.
***********************************************************************
and it often APPEARS dead 'cos no one transmits! So good luck to Ed with
his 10m attempts. Get CQ'ing and you may suddenly get some activity
going. 10 is a fantastic band when the sun gets active and I'm looking
forward to this again in a few years time.
73, Peter G4BVH
Don
Sounds to me like you have a short. Check the center conductor to the
shield with an ohmmeter. Should be infinity. W5BB
arm...@delphi.com
Alexander Vrenios <vre...@enuxsa.eas.asu.edu> writes:
>>> : coax cable but I cannot get the SWR reading below 6:1. Even with an
>>> : antenna tuner I cannot get an acceptable SWR reading. It's driving me to
that goes into the rig to the end of the antenna wire. Then check for continuity
from the barrel of the coax connector to the end of the opposite wire of the
dipole. Then check for a short between the pin and the barrel. Just about any
tuner
would match up a 6:1 mismatch. Should you still be in a quandry after all
the good advice you have received....if you can be at your rig/tuner during
the day, call me on the phone and I'll try to tsalk you through the tuneup
procedure...........(916) 457-3655 ask for armond.
Ross Archer
Chr...@valteck.com (Chris Valliant) wrote:
> I have been trying for the last two weeks to build a half wave
>diapole antenna. I have read the book twice, done my calculations down to
>the half inch, used several different gauges of wire, several makes of
>coax cable but I cannot get the SWR reading below 6:1. Even with an
>antenna tuner I cannot get an acceptable SWR reading. It's driving me to
>the funny farm. I have about a mile of wire in my apartment and have run
>out of ideas. Here is what I am trying to do: to build a 1/2 wave
>diapole for the 28-29 mhz. I splice the coax, solder one end to a length
>of wire 8.2 feet and the other braided end to a length of wire the same
>size. I suspend both ends using insulated clamps between two tree's,
>making sure that the whole antenna is parallel to the ground and it's 8
>feet above the ground. Now when I check the SWR, it's unbelieveable. Like
>I have said before, I have used different wire, different coax (the
>standard coax, just different brands). What am I doing wrong ? Can anyone
> spot my mistake ? I will gladly make it up to whomever spots my error.
>Anyway, thanks for listening.
> 73's
> Chris Vallliant
You need to put your antenna higher or go to a vertical that works
against the ground if you can't raise it to at least 1/2 wavelength
high.
First, 8 feet off the ground is barely 1/4 wavelength off the ground
at 10 meters. This is doing three bad things:
1. It throws off (reduces) the impedance of the antenna
feedpoint, because the ground is partially conductive
and in the antenna's near-field. This is bad, because
a dipole is balanced and doesn't need a ground to work
against. And obviously, this raises the SWR.
2. The ground actually will absorb power from your transmitter.
It's the "heat the earthworm" factor.
3. It messes up your radiation pattern. It will look more like an
isotropic antenna and put a lot of energy at high angles.
At 10 meters, this you do not want.
That being said, I doubt your SWR is actually 6:1.
I read in the Radio Amateurs Handbook that you
need a balun at the antenna feedpoint, or otherwise
strong currents will flow on the outer conductor, pretty
much equal to the current flowing in the inner conductor.
For some reason I don't understand, this "confuses"
SWR meters and other equipment since the shield
is not supposed to carry current.
So, try the balun, and raise the antenna another 8 feet, and
I bet you'll get a much better SWR.
Just my $0.02 worth.
Ross
Chris Valliant
In article <8C3A46D.0407...@cheaha.com>, ed.w...@cheaha.com
says...
>
>-> Here is what I am trying to do: to build a 1/2 wave >diapole for the
>-> 28-29 mhz. I splice the coax, solder one end to a length
>
>
>-> 5) Why 28-29 MHZ? The band's dead.
>
>Oh, not completely dead. I've been picking up several QSO's for several
>days now. 'Caught a NE net the other evening, and a guy CQing out of
>Quebec. Waiting on my ticket to arrive I haven't responded to any of
>these. Maybe crippled, but certainly not dead.
>
>Chris might be like me, just getting his feet wet in radio and wanting
>skilled operator. Digging weak signals out now could help in the
>future.
>
>I agree that 10m ain't hopping, but as I write this I'm copying a guy in
>northern Ohio on 28.47010...I'm down in south Alabama. Worldwide, no,
>but workable.
>
>Just my two cents worth. :)
>
>73
>ed.w...@cheaha.com
That,s true ed. I haven't been using the radio for longer than
three months if that. But hey, you have got to start somewhere. While I
would love to go out an plunk down $10,000 on a complete system, that just
isn't in my budget. Beside's, I just love building things :) Whether they
work or not is besides the point :) But it is true, the 10M is comming
back in, I can hear more and more each night. Some of it is rather faint,
but it is getting alot better.
Anyway, thanks alot for the input. Perhaps we will meet someday :)
Chris Valliant